Explicitly set default encoding in createHash()

Up until now, the default encoding for this is `binary`, but
this is scheduled to change in Node 8. Since this module is
currently always passing string input to the `Hash` object,
it needs to account for that.

A better fix would likely involve dropping all strings handling
and treating binary data using Buffer objects, but I kept
this change minimal to avoid any breakage.

Refs: nodejs/node#8611

index.js

@@ -138,7 +138,7 @@ class ChecksumValidator {
       let fullPath = path.resolve(baseDir, filename)
       debug(`Reading file with "${this.encoding(binary)}" encoding`)
       let stream = fs.createReadStream(fullPath, {encoding: this.encoding(binary)})
-      let hasher = crypto.createHash(this.algorithm)
+      let hasher = crypto.createHash(this.algorithm, {defaultEncoding: 'binary'})
       hasher.on('readable', () => {
         let data = hasher.read()
         if (data) {